1. Field of the Invention
The present invention relates to photo-alignment materials useful in liquid crystal alignment films. More particularly, the present invention relates to photo-alignment materials useful in liquid crystal alignment films in which the pretilt angle of the materials is freely controllable while providing a display quality equivalent or superior to alignment materials made using the conventional rubbing process.
2. Description of the Related Art
The arrangement of liquid crystals in a liquid crystal display device changes in accordance with an electric field induced by an externally applied voltage. Such changes in the alignment of the liquid crystals determine whether external light entering the liquid crystal device is blocked or transmitted. Thus, the liquid crystal device can be driven by this property of the liquid crystals. The quality of a liquid crystal display device as a display device is determined by properties that are varied according to the alignment state of the liquid crystals, including light transmittance, response time, viewing angle, contrast ratio, and the like. Therefore, it is very important to uniformly control the liquid crystal alignment in liquid crystal devices.
An alignment film typically refers to a layer of polymer material, which is formed between liquid crystals, and a transparent conductive film made of indium tin oxide in order to produce the uniform alignment, i.e., orientation of liquid crystalline molecules. After formation, the polymer layer typically is subjected to a mechanical process, such as rubbing and the like, and other processes to control the alignment of liquid crystals.
The method currently used to achieve uniform alignment of liquid crystals or to orient liquid crystals in a given direction in preparing liquid crystal display devices involves disposing a layer of polymer, such as polyimide, on a transparent conductive glass substrate, and then rubbing the polymer surface at a high speed with a rotating roller that is wrapped with a rubbing cloth made of nylon or rayon. By this rubbing process, the liquid crystalline molecules are oriented with a specific pretilt angle on the surface of the alignment film.
Since this rubbing process is substantially the only method to orient liquid crystals easily and stably, most manufacturers producing liquid crystal display devices generally use the rubbing process for mass-production. However, the rubbing process has problems in that it creates scratches on the surface of the liquid crystal alignment film due to mechanical rubbing, and it generates static electricity which leads to destruction of thin film transistors. In addition, micro fibers released from the rubbing cloth may cause defects in the liquid crystal devices. Accordingly, this rubbing process reduces the production quality of the devices. A new alignment technique has been proposed that aligns liquid crystals by irradiation of light, for example, UV rays, in order to overcome the problems involved in the rubbing process as described above, and thereby to improve productivity.
Recently, liquid crystal displays have become large-scale, and the applications of the liquid crystal display are expanding beyond personal applications, such as notebook computers, to household applications, such as wall-mounted TVs. In accordance with this trend, a high quality picture and a wide viewing angle are required for the liquid crystal display devices. Also, in order to meet such demands for qualities of the liquid crystal display, the photo-alignment method is currently in the spotlight.
However, the photo-alignment methods reported by M. Schadt et al. (Jpn. J. Appl. Phys., Vol. 31, 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys., Vo. 34, 1992, L1000) are not yet commercialized, in spite of the superiority of its concept, because there is difficulty in developing novel materials to support these methods. One of the major reasons for the difficulty is that raw materials of the alignment films are not sufficiently processible to be applied to the conventional method for manufacturing liquid crystal display devices. Also, the display device using the alignment film formed by photo-alignment is inferior in display quality, as compared to the display device formed using an alignment film of polyimide by rubbing process.
Currently, the display quality of the liquid crystal display devices is increasingly enhanced and these liquid crystal display devices are recognized as display devices having the best picture quality. There also are efforts to enhance color reproducibility for the purpose of promoting the development of the liquid crystal display devices in various fields. The color reproducibility can be facilitated by improving the function of the alignment film. To this end, appropriate control of the pretilt angle of liquid crystals has become the focus of much research. So far, the pretilt angle of liquid crystals has been increased from a level of from about 1xcx9c3xc2x0 to a level of from about 3xcx9c5xc2x0. In order to realize more natural color, it is desirable to raise the pretilt angle to about 7xc2x0 or higher.
Typically used alignment materials made using the rubbing process, however, have a pretilt angle of from about 3xcx9c5xc2x0. When the pretilt angle is higher or lower than the aforementioned values, the aligning character is weakened relatively or faults may occur due to scratches on a surface. More specifically, though the pretilt angle is increased, it is difficult to attain a stable pretilt angle throughout the entire surface of a screen and partial nonuniformities may be observed. In addition, it is considered to be very difficult to increase the pretilt angle without deterioration of other display quality. Thus, new materials that satisfy the above conditions are required.
The disadvantages and deleterious properties described above with reference to certain materials, devices, methods, and apparatus is not intended to limit the present invention. Indeed, certain features of the invention may include any or all of the materials, devices, methods, and apparatus, without suffering from the disadvantages and deleterious properties so described.
A feature of an embodiment of the present invention is to provide a novel alignment material useful in a liquid crystal alignment film in which the pretilt angle of the alignment material is freely controllable within the range of from about 1xcx9c10xc2x0. The materials of the invention provide a display quality equivalent or superior to the alignment materials made using the conventional rubbing process.
In accordance with one feature of an embodiment of the present invention, there are provided photo-alignment materials useful in liquid crystal alignment films comprising a repeating unit represented by the following formula 1 and at least one repeating unit selected from the structures represented by the following formula 8, wherein at least 20% of the repeating units contain at least one photo-reactive functional group selected from the structures represented by the following formula 5: 
in which X is a hydrogen atom, fluorine atom, chlorine atom, or C1xcx9c18 linear or branched alkyl group; Y is an oxygen atom or C2-18 alkylene group; and R is a functional group having a structure represented by the following formula 3:
"Parenopenst"R1"Parenclosest"k"Parenopenst"R2"Parenclosest"I"Parenopenst"R3)xe2x80x83xe2x80x83(3)
in which R1 is at least one of the functional groups represented by the following formula 4; R2 is at least one of the functional groups represented by the following formulas 5 and 6; R3 is at least one of the functional groups represented by the following formula 7; k is an integer of from 0 to 3; I is an integer of from 0 to 5; and if there exist a plurality of R1 or R2, each R1 or R2 may be same or different: 
in which n is an integer of from 0 to 10, 
in which X in the functional groups represented by formula (7) is a hydrogen atom, fluorine atom, chlorine atom, C1xcx9c13 alkyl or alkoxy group, or xe2x80x94(OCH2)pCH3 in which p is an integer of from 0 to 12, and m is an integer of from 0 to 18; 
in which n is an integer of from 1 to 12.
According to another feature of an embodiment of the present invention, there are provided photo-alignment materials useful in liquid crystal alignment films comprising a repeating unit represented by the following formula 1, at least one repeating unit selected from the structures represented by the following formula 2, and at least one repeating unit selected from the structures represented by the following formula 8, wherein at least 20% of the repeating units contain at least one photo-reactive functional group selected from group consisting of structures represented by the following formula 5: 
in which X is a hydrogen atom, fluorine atom, chlorine atom, or C1xcx9c14 linear or branched alkyl group; Y is an oxygen atom or C2xcx9c14 alkylene group; and R is a functional group having a structure represented by the following formula 3:
"Parenopenst"R1"Parenclosest"k"Parenopenst"R2"Parenclosest"I"Parenopenst"R3)xe2x80x83xe2x80x83(3)
in which R1 is at least one of the functional groups represented by the following formula 4; R2 is at least one of the functional groups represented by the following formulas 5 and 6; R3 is at least one of the functional groups represented by the following formula 7; k is an integer of from 0 to 3; I is an integer of from 0 to 5; and if there exist a plurality of R1 or R2, each R1 or R2 may be same or different: 
in which n is an integer of from 0 to 10, 
in which X in the functional groups represented by formula (7) is a hydrogen atom, fluorine atom, chlorine atom, C1xcx9c13 alkyl or alkoxy group, or xe2x80x94(OCH2)pCH3 in which p is an integer of from 0 to 12, and m is an integer of from 0 to 18; 
in which n is an integer of from 1 to 12.